1. Field of the Invention
This invention relates to control of voltage supplied to thermal printheads and in particular to pulse width modulation of such voltage.
2. Description of the Prior Art
Driving a thermal printhead requires maintenance of an adjustable temperature while printing, despite variations in character density and ambient temperature.
In the prior art, it is common to use a linear regulation scheme for providing the power to the printhead, and a feedback and comparison loop to adjust the temperature. A series pass transistor is employed to apply power to the thermal printhead. However, the use of such a transistor necessarily wastes approximately one half of the supplied power.
To overcome this problem, a switching regulator power supply has been employed to provide a controlled voltage to the thermal printhead. Such a switching regulator for a thermal printhead is described and claimed in copending U.S. patent application Ser. No. 146,992 filed May 5, 1980 and assigned to the assignee of this invention. This use of a switching regulator to supply voltage to a thermal printhead results in a significantly more efficient means of supplying power.
In the present invention, instead of supplying a regulated voltage to the thermal printhead, a pulse train is applied and such pulse train is effectively regulated. The result is an efficient, low cost regulation circuit.